1. Field of the Invention
The present invention relates to a display controller for displaying characters, graphics, and symbols (hereafter referred to as characters) on a television screen.
2. Description of the Prior Art
It is conventionally performed to display channels and characters and patterns expressing various operating states on a television screen. FIG. 6 is a block diagram showing this type of the existing display controlled.
Horizontal synchronizing signals are input to the H (horizontal) counter 1 and the H counter 1 is reset by each vertical synchronizing signal for changing the screen. The value counted by the H counter 1 serves as a criterion for determining the position of a character or pattern to be displayed on the screen.
Code data and address data to be displayed are input to the input control circuit 2 from a CPU not illustrated.
Meanwhile, in FIG. 5, the television screen T is sectioned into squares consisting of "j" rows and "i" columns (j.times.i) for display of characters or the like, which are set so that one basic character will be set to each square P. For an actual television screen, characters to be displayed use the bottom or top portion of the screen T and the entire screen T is rarely filled with characters. Therefore, some of squares P among those consisting of "j" rows and "i" columns are frequently filled with characters.
The write control circuit 6 divides input data into address data according to the sequence of the above squares P and data for the code of character or blank (non-character) to be set to the address and records the data in the display data RAM7 in order of address.
Because the height of each square P is specified by the number of horizontal synchronizing signals (raster), the horizontal synchronizing signal is also given to the write control circuit 6.
In FIG. 5, the case is assumed in which input data values for specifying characters A and B are set to the fifth and sixth squares P from the top left, those for specifying characters Y and Z are set to the second and first squares P from the bottom right, and non-characters are set to other squares.
The write control circuit 6, as shown in FIG. 7, records characters and non-characters (blanks) in the display data RAM7 so that squares P will correspond to addresses "a", "a+1", . . . , and "a+i j-1" one each.
Numeral 9 is a character ROM which outputs the character font read according to the character code recorded in the RAM7. The read control circuit 13 synchronizes with the horizontal synchronizing signal to read the character codes from the RAM7 and send them to the display control circuit 11. The display control circuit 11 displays the character font and non-character (blank) signal sent from the character ROM9 at the specified screen position of the CRT14. Video signals are also input to the CRT14 from a television circuit not illustrated and the character font or the like superimposed on the picture is displayed.
Because a semiconductor device having the conventional display control circuit is configured as shown above, it stores a not-displayed character (blank) position in the memory on a RAM by making it correspond to the data for one character. Therefore, there is the problem that a large-capacity display data RAM is required even for a system actually having the small number of display characters on the screen.